[v16] update mfa missing error (#46678)

* update mfa add error

* docs: update verbiage for missing MFA

Co-authored-by: rosstimothy <39066650+rosstimothy@users.noreply.github.com>

---------

Co-authored-by: Steven Martin <stevenmartin@stevens-mbp.lan>
Co-authored-by: rosstimothy <39066650+rosstimothy@users.noreply.github.com>

lib/auth/authclient/clt.go

@@ -78,7 +78,7 @@ var (
 	// ErrNoMFADevices is returned when an MFA ceremony is performed without possible devices to
 	// complete the challenge with.
 	ErrNoMFADevices = &trace.AccessDeniedError{
-		Message: "MFA is required to access this resource but user has no MFA devices; use 'tsh mfa add' to register MFA devices",
+		Message: "MFA is required to access this resource but user has no MFA devices; see Account Settings in the Web UI or use 'tsh mfa add' to register MFA devices",
 	}
 	// InvalidUserPassError is the error for when either the provided username or
 	// password is incorrect.